Minor changes to Python source base loader.
Minor changes to Python source base loader.

Fixed a bug where 'self' was left off a method call. Was masked by the fact the
source/bytecode loader subclass is always used. Cleaned up when the source path
is fetched. Also made sure ImportError is raised when a source path cannot be
found.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator